It has heretofore been proposed to produce ultra-high purity nitrogen in an air separation cycle by separating nitrogen from air and condensing the nitrogen. Uncondensed gas is discharged and contains large quantities of low boiling point substances. The liquid nitrogen containing reduced quantities of the low boiling point substances is introduced as reflux to the top of a fractionating column, and a large part of the liquid nitrogen introduced as reflux is removed from the column from a fractionating plate located several plates below the top plate of the column.
The nitrogen product thus produced has progressively reduced quantities of low boiling point substances, the lower is the fractionating plate from which the liquid nitrogen introduced as reflux is taken. However, this technique has the disadvantage that oxygen, which of course is a somewhat higher boiling point substance, tends to remain as an impurity in the nitrogen product; and the degree of purity of the nitrogen product thus suffers.